No abstract

GaSb-based type-I quantum-well diode lasers emitting at 3.36 m at 12°C with 15 mW of continuous wave output power are reported. Devices with two or four InGaAsSb compressively strained quantum wells and AlInGaAsSb quinternary barriers were fabricated and characterized. It was shown that increase in the quantum-well number led to improved laser differential gain and reduced threshold current.The growing demand for light emitters capable of high power room temperature operation in spectral region from 3 to 3.5 m encourages intensive search for appropriate design approach to fabricating such photonic devices. Both monopolar and bipolar lasers based on cascade and multiple quantum well ͑QW͒ active region designs are being extensively researched for this purpose ͑see, for instance, Refs. 1 and 2͒. GaSb-based type-I QW diode lasers operate in continuous wave ͑cw͒ at room temperature in spectral region above 3 m. 3,4 Diode lasers grown by molecular beam epitaxy on GaSb substrates and operating up to 3.1 m at room temperature in cw mode have been recently reported by our group. 5 Further increase in the operating wavelength of type-I QW GaSb-based lasers faces two major complications ͑a͒ gradual decrease in the valence band offset between InGaAsSb QW and Al containing barrier layers when In and As concentrations in QW are increased 6 and ͑b͒ possible increase in the nonradiative Auger recombination rate when QW bandgap decreases. The latter is thought to be fundamental in nature.The lack of valence band offset in GaSb-based heterostructures can be overcome by introduction of heavy compressive strain above 1% into InGaAsSb QWs 7 and utilization of the quinternary AlInGaAsSb barrier material. 4,8 The contribution of the Auger recombination processes to threshold current can be minimized by a reduction in the threshold carrier concentration through improvement of the hole confinement itself and by increase in the number of QWs in device active region.In this work we report on the development of the 3.36 m emitting lasers with strained active region and quinternary barrier material. We demonstrate that increase in the number of QWs from two to four decreases the device threshold current through improvement in the laser differential gain ͑with respect to current͒. Diode lasers with four QW active regions operate in cw mode at 12°C with 15 mW of output power at 3.36 m.Laser heterostructures were grown at State University of New York at Stony Brook by solid-source molecular beam epitaxy using Veeco GEN-930 reactor equipped with As and Sb valved cracker sources. Te and Be were used for nand p-doping, respectively. Laser active region contained either two or four 16 nm wide 1.5% compressively strained InGaAsSb QWs with nominal In composition of 54%. The interwell spacings were 40 and 20 nm in two-QW and four-QW devices, respectively. The waveguide ͑total width of 1 m͒ and barrier materials were Al 0.20 In 0.25 Ga 0.55 As 0.26 Sb 0.74 . The cladding material was Al 0.9 Ga 0.1 As 0.07 Sb 0.93 . Graded bandgap heavily doped transiti...

A continuous-wave, room temperature operation of type-I quantum well diode lasers was extended above 3.4 mm. The laser heterostructure, optimised for minimum threshold carrier concentration, was pseudomorphically grown by solid source molecular beam epitaxy on GaSb. Multimode lasers generate 29 mW of output power at 17 8C.Introduction: Lasers emitting in the spectral region from 3 to 3.5 mm are in demand for a variety of applications ranging from trace gas spectroscopy to infrared countermeasures. Room temperature continuouswave (CW) operated diode lasers are especially attractive thanks to their compactness, robustness, low operating voltage and potentially low cost. Corresponding devices can be spectrally stabilised either internally or externally and then utilised in spectroscopic sensors, for instance, to monitor methane concentration in atmosphere [1].Until recently, the very idea of CW diode lasers operating at room temperature with a wavelength above 3 mm was thought to be questionable. This pessimism originated in the well-known fundamental increases of the non-radiative Auger recombination and of free carrier absorption with wavelength. The associated carrier and photon losses were considered likely to prevent the mid-infrared diode lasers from reaching CW lasing without cryogenic cooling. Today, diode lasers operating at 3 mm not only reach CW threshold but also produce a hefty 300 mW of output power and demonstrate threshold current densities of 200 A/cm 2 [2], i.e. only twice worse than those of highly efficient 1 mm diode lasers [3]. Clearly, this experimental fact casts doubt on the validity of the old argument based on the destructive power of Auger recombination and free carrier absorption. Moreover, the narrow bandgap nature of the active region not only brings disadvantages but can also provide significant benefits. The advantage of using narrow bandgap material for generation of optical gain is rather straightforward though rarely discussed in the context of mid-infrared laser development. The electron density of states in the G-valley is nearly proportional to the direct material bandgap in III-V binaries [4]. This proportionality is more or less preserved in corresponding ternary and quaternary alloys used in quantum wells (QWs) of diode lasers. In QWs compressively strained above 1%, the top valence subband has in-plane effective mass that is rather close to electron effective mass [5]. The resulting reduced density of states facilitates separating the quasi-Fermi levels in narrow bandgap active QWs under injection. Since the bulk transition matrix element does not show pronounced material bandgap dependence [4], this leads to improved differential gain in narrow bandgap QWs. Hence, it is expected that mid-infrared diode lasers can have much lower transparency and, possibly, lower threshold carrier concentrations as compared to their near-infrared counterparts. Auger recombination is a three particle process and its net rate is superlinear in carrier concentration. Hence, threshold current d...